"It turns out that when you install Lion, the installer creates an invisible, bootable, 650MB partition--a portion of a drive the operating system treats as a separate volume--on your startup drive called Recovery HD that includes a few essential utilities for fixing problems, restoring files, browsing the Web, and even reinstalling Lion."

"The idea behind recovery mode is that if you ever have problems with your Mac's startup volume, you can boot from Recovery HD and perform some basic troubleshooting procedures without the need for an OS X installation DVD..."

BTW, I already tested the clean install and the recovery methods. Pretty neat, but I advise users to make and keep a Lion boot disk or flash drive handy for those days when nothing goes right - like not being able to access your hard drive.

A number of appearance and functional things in Lion are different than in Snow Leopard.* A few that may be especially disconcerting or confusing to those accustomed to earlier versions of OSX. Here is a list of them and how to revert if you need to:Apple Tips Some Changes in Lion

Lion introduced a new feature called "auto-resume" where apps you close will reopen right where you left off. Many users find this feature annoying, and it's not obvious where and how to turn it off.

Here is how to turn off auto-resume.

Go into System Preferences and select the General tab. At the bottom of that preferences pane is a checkbox labeled "Restore windows when quitting and re-opening apps." If you uncheck that, your apps will return to their old Snow Leopard behavior.